Rotating electrical machine with electromagnetic and permanent magnet excitation

ABSTRACT

In several embodiments of the present invention, an alternator is disclosed. The alternator includes a rotor having both wound-field and permanent magnet poles. The wound-field and permanent magnet poles are disposed about the circumference of the rotor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to rotating electrical machines.

2. Description of the Related Art

One of the challenges in the design of electrical systems forautomobiles relates to the generation of electrical power. With theincreasing number of electrical features on automobiles comes the needfor increased electric power generation. Thus, one challenge is to findan alternator which can provide this increased power with a minimum ofadverse vehicle-level consequences (such as increased cost, increasedmass, increased packaging volume, and the like).

Further, because the power consumption of most electrical loads on anautomobile is independent of engine speed, the aforementioned increasedneed for electrical power exists down to engine idle. Because the poweroutput of alternators is generally a function of the speed at which theyare driven, a large electrical load at idle can be particularlydifficult to supply.

One proposed solution to the above concerns is a so-called "hybrid"alternator. Such an alternator would contain a rotor which combinesfield windings (a conventional feature of almost all presentalternators) with added permanent magnets. Several patents disclosehybrid alternators, including U.S. Pat. No. 3,555,327, issued to Terry;U.S. Pat. No. 4,980,595, issued to Arora; U.S. Pat. Nos. 4,882,515 and4,959,577, issued to Radomski; U.S. Pat. Nos. 5,132,581 and 5,177,391,issued to Kusase; and U.S. Pat. No. 5,397,975, issued to Syverson.

Although those designs just mentioned may have desirable features, otherdesigns may provide superior power output. Just as importantly,alternative designs may prove to be more manufacturable than thosedesigns. Thus, an alternator which can provide improved power output,particularly at low speeds, and which can do so in a highlymanufacturable design, will provide advantages over the prior art.

SUMMARY OF THE INVENTION

The present invention provides an electrical machine. The electricalmachine includes a generally-annular stator and a rotor rotateablymounted within the stator. The rotor comprises a plurality ofwound-field poles disposed about at least one portion of thecircumference of the rotor and a plurality of permanent magnet poles alldisposed about at least one circumferentially-shifted portion of thecircumference of the rotor. The permanent magnet poles each further havea magnetized surface of one polarity generally facing the stator and amagnetized surface of the opposite polarity generally facing away fromthe stator.

The present invention further provides another electrical machine. Theelectrical machine comprises a generally-annular stator and a rotorrotateably mounted within the stator. The rotor includes a plurality ofwound-field poles disposed about at least one portion of thecircumference of the rotor and a plurality of permanent magnet poles alldisposed about at least one circumferentially-shifted portion of thecircumference of the rotor. The wound-field poles are disposed incircumferentially-consecutive pairs.

The present invention additionally provides an electrical machinecomprising a generally-annular stator and a rotor rotateably mountedwithin the stator, the rotor comprising a plurality of wound salientpoles. The wound salient poles each further comprise a base and awinding about the base. At least some of the bases of the salient polesare substantially parallel to one another.

One embodiment of this invention is an alternator which has thepotential to provide improved power output, particularly at low speeds,over other alternative alternator designs. This alternator is alsohighly manufacturable, an advantage over many other designs. For thesereasons, the present invention can provide advantages over the priorart.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a rotor 10 of an electrical machineaccording to one embodiment of the present invention.

FIG. 2 is a partial cross-sectional view of rotor 10, showing in detailpermanent magnet poles 12 and 14. FIG. 2 also shows a partialcross-section of stator 74 of an alternator 75 which comprises rotor 10and stator 74.

FIG. 3 is a partial cross-sectional view of rotor 10, showing in detailwound-field poles 36 and 38.

FIG. 4 is a block diagram showing a voltage regulator 80 for controllingthe output voltage of alternator 75.

FIG. 5 is an exploded view of pertinent portions of a rotor 110according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a rotor 10 for an electrical machine accordingto one embodiment of the present invention will be described. Rotor 10is rotateably mounted on a shaft 11. In this embodiment of the presentinvention, rotor 10 has 12 magnetic poles, though the invention is notso limited. Six of the poles 12, 14, 16, 18, 20 and 22 are formed bypermanent magnets 24, 26, 28, 30, 32 and 34. The remaining six poles 36,38, 40, 42, 44 and 46 are wound poles. Those skilled in the art willrecognize those wound poles as "salient" poles, as opposed to "clawpoles" in Lundell-type rotors.

Poles 36, 38, 40, 42, 44 and 46 are magnetized by field windings. Thosewindings include winding 50, wound around base 52 of pole 36; winding54, wound around base 56 of pole 38; winding 58, wound around base 60 ofpole 40; winding 62, wound around base 64 of pole 42; winding 66, woundaround base 68 of pole 44; and winding 70, wound around base 72 of pole46. In the preferred embodiment of the present invention, windings 50,54, 58, 62, 66 and 70 are all connected in series. However, this seriesconnection is not an essential feature of the present invention.

Windings 50, 54, 58, 62, 66 and 70 are wound such that for each adjacent(that is, consecutive) pair of wound poles (e.g., 36 and 38; 40 and 42;or 44 and 46), the windings on the two adjacent poles are wound inopposite directions. Thus, for a given direction of current flowing inthe field coil comprising windings 50, 54, 58, 62, 66 and 70, theadjacent poles in each pair of poles will have opposite magneticpolarities.

With respect to permanent magnet poles 12, 14, 16, 18, 20 and 22, thesepoles are likewise adapted such that for each adjacent (that is,consecutive) pair of poles (e.g., 12 and 14; 16 and 18; or 20 and 22)the two poles have opposite magnetic polarities. The magnetic polarityof each permanent magnet pole is the magnetic polarity of theradially-outward surface of the permanent magnet within the pole (e.g.,permanent magnet 24 within pole 12).

In the preferred embodiment of the present invention, permanent magnets24, 26, 28, 30, 32 and 34 are rectangular in cross-section. They arepreferably neodymium-iron-boron (NdFeB) or other rare earth material,though ceramic or other permanent magnet material can also be used toadvantage in this invention.

Rotor 10 is preferably constructed of iron laminations, each punchedwith the cross-sectional features shown in FIG. 1. When the laminationsare stacked together, the rectangular pockets for permanent magnets 24,26, 28, 30, 32 and 34 are formed. Once the permanent magnets areinserted into their respective pockets, a final non-magnetic laminationwithout openings for permanent magnets 24, 26, 28, 30, 32 and 34 can beadded on each end of rotor 10. These final laminations will hold thepermanent magnets in place. Alternatively, adhesives or potting materialcan be used to hold the permanent magnets in place in their respectivepockets in rotor 10.

Although rotor 10 is preferably constructed of laminations, the presentinvention is not so limited. By way of example, rotor 10 can also bemade solid, as opposed to laminated.

Referring now to FIG. 2, further features of rotor 10 will be described.FIG. 2 also illustrates stator 74, within which rotor 10 is rotateablymounted. Rotor 10 and stator 74 (among other components such as sliprings, a housing, cooling fans and the like) comprise an alternator 75.FIG. 2 illustrates permanent magnet poles 12 and 14, comprisingpermanent magnets 24 and 26, respectively. Most of the magnetic fluxgenerated by permanent magnets 24 and 26 is disposed as shown by fluxlines 76. As is illustrated, most of the flux is not located in base 78of poles 12 and 14. As a result, base 78 has little tendency towardmagnetic saturation, thus requiring less iron per pole than the bases ofthe wound-field poles. Advantage is taken of the lower iron requirementin designing the windings of wound-field poles 36, 38, 40, 42, 44 and46, as shown in FIG. 3.

FIG. 3 illustrates wound-field poles 36 and 38 (stator 74 is omitted forclarity). Because the bases of the permanent magnet poles requirerelatively little iron, bases 52 and 56 of wound-field poles 36 and 38can be made parallel to one another. In conventional rotor designs,bases 52 and 56 would likely be radially-oriented, instead of parallelto one another. If bases 52 and 56 were radially-oriented, the spacessurrounding them would taper down near the center of rotor 10, leavinglittle room for windings in that area. However, being parallel to oneanother, bases 52 and 56 allow a very large amount of wire to be woundaround them, even near the center of rotor 10. This high density of wiregives alternator 75 a high power density (i.e., power output per unitvolume of alternator 75).

Control of the field current in rotor 10 is preferably accomplished bymeans of a bi-directional current regulator. One such regulator 80 isshown in FIG. 4. Regulator 80 comprises four semiconductor switches 82,84, 86 and 88, arranged in an "H"-bridge configuration between systemvoltage (V_(sys)) and ground. The series connection of windings 50, 54,58, 62, 66 and 70 is shown as field coil 89. Control circuitry 90 isconnected to the gates of each semiconductor switch. Control circuitry90 is also provided with a sense input connected to system voltage, inorder to have feedback information regarding system voltage.

Control circuitry 90 can use conventional negative-feedbackproportional-integral (PI) control to modulate the current through fieldcoil 89, thereby regulating the output voltage of alternator 75. Thedeparture of the design of regulator 80 from conventional regulatordesign is that bi-directional current is to be controlled through fieldcoil 89 via switches 82, 84, 86 and 88. The reason for suchbi-directional control will become apparent during the upcomingdiscussion.

Referring to FIGS. 1 and 4, the operation of alternator 75 will now bediscussed. With current flow in one direction (say, direction 92) infield coil 89, all poles of rotor 10 will alternate in magnetic polarityaround the circumference of rotor 10. Beginning with pole 12 andprogressing clockwise in FIG. 1, and assuming pole 12 to be a northmagnetic pole, the polarity of the poles of rotor 10 will be as follows:

                  TABLE 1                                                         ______________________________________                                                Pole Polarity                                                         ______________________________________                                                12   north                                                                    14   south                                                                    36   north                                                                    38   south                                                                    16   north                                                                    18   south                                                                    40   north                                                                    42   south                                                                    20   north                                                                    22   south                                                                    44   north                                                                    46   south                                                            ______________________________________                                    

With these polarities, wound-field poles 36, 38, 40, 42, 44 and 46 workin an additive manner with permanent magnet poles 12, 14, 16, 18, 20 and22 to "boost" the output of alternator 75. If, for a given speed ofrotation of rotor 10, the output voltage of alternator 75 is too high,regulator 80 will reduce the average current through field coil 89. Thisreduction in current will reduce the flux generated by the wound-fieldpoles of rotor 10, thus reducing the output voltage from alternator 75.

However, for some rotational speeds of rotor 10, even reducing currentflow in field coil 89 to zero can still result in output voltages abovewhich regulator 80 is attempting to regulate. This would be true becauseof permanent magnet poles' 12, 14, 16, 18, 20 and 22 ability to generatemagnetic flux with no field current in field coil 89. If a reduction tozero field current is not sufficient, regulator 80 will cause currentflow in the opposite direction (say, direction 94) in field coil 89. Bymodulating the current in the opposite direction, regulator 80 willreduce the output voltage of alternator 75 to the target value.

When regulator 80 changes the direction of the current through fieldcoil 89, the voltage induced in the windings of stator 74 "bucks" thevoltage induced by the permanent magnet poles. In this event, thepolarities of the poles of rotor 10 will be as follows:

                  TABLE 2                                                         ______________________________________                                                Pole Polarity                                                         ______________________________________                                                12   north                                                                    14   south                                                                    36   south                                                                    38   north                                                                    16   north                                                                    18   south                                                                    40   south                                                                    42   north                                                                    20   north                                                                    22   south                                                                    44   south                                                                    46   north                                                            ______________________________________                                    

As a comparison of Table 2 with Table 1 illustrates, each of thewound-field poles 36, 38, 40, 42, 44 and 46 changed polarity when thedirection of current through field coil 89 changed. The polarity ofpermanent magnet poles 12, 14, 16, 18, 20 and 22, of course, remainedunchanged.

The design of rotor 10 of this embodiment of the present invention ishighly manufacturable, largely because it is of uniform cross-sectionfor its entire axial length. Rotor 10 is thus a single rotor, as opposedto designs which are really two separate rotors which are axiallymarried. For the same reason, rotor 10 can be made axially shorter thanrotors of other such designs. This can provide packaging advantages.

As has been discussed, the preferable design for rotor 10 includes sixpermanent magnet poles disposed in pairs and six wound-field poles alsodisposed in pairs. However, the present invention is not limited to sucha number or configuration of poles. For example, the number ofwound-field poles to be employed can vary based on the speed range inwhich alternator 75 is to be operated. If operating in a narrower speedrange than the alternator of FIG. 1, fewer wound field poles may berequired in order to sufficiently "buck" the voltage induced by thepermanent magnet poles at the upper end of the speed range. In such acase, four wound-field poles (for example) might be employed. In thatevent, the poles of rotor 10 may have polarities as follows with currentin one direction in field coil 89 (with "PM" referring to a permanentmagnet pole and "WF" referring to a wound-field pole):

                  TABLE 3                                                         ______________________________________                                               Pole Type                                                                            Polarity                                                        ______________________________________                                               PM     north                                                                  PM     south                                                                  PM     north                                                                  WF     south                                                                  WF     north                                                                  PM     south                                                                  PM     north                                                                  PM     south                                                                  WF     north                                                                  WF     south                                                           ______________________________________                                    

Here, the wound-field poles would "boost" the alternator outputgenerated by the permanent magnet poles. With current in the otherdirection in field coil 89, the poles would be configured as follows:

                  TABLE 4                                                         ______________________________________                                               Pole Type                                                                            Polarity                                                        ______________________________________                                               PM     north                                                                  PM     south                                                                  PM     north                                                                  WF     north                                                                  WF     south                                                                  PM     south                                                                  PM     north                                                                  PM     south                                                                  WF     south                                                                  WF     north                                                           ______________________________________                                    

Here, the voltage induced by the wound-field poles will "buck" thevoltage induced by the permanent magnet poles.

An alternative design for a rotor 110 according to another embodiment ofthe present invention is illustrated in FIG. 5. Here, rotor 110 is in aLundell or "claw pole" configuration. Rotor 110 comprises two polepieces 112 and 114, disposed on a shaft (not shown) and defining an axisof rotation 115 of rotor 110. Disposed on the hubs of the pole pieces isa single field coil 116. In a conventional Lundell alternator, all ofthe pole fingers 118 through 140 would be magnetized by field coil 116(and would thus be "wound-field" poles). Pole fingers 118 through 128 ofpole piece 112 would be magnetized with one magnetic polarity, and polefingers 130 through 140 of pole piece 114 would be magnetized with theopposite magnetic polarity.

In this embodiment of the present invention, however, some of the polefingers are replaced by permanent magnets poles. Such permanent magnetpoles preferably have a surface of one polarity generally facing thestator of the alternator, and a surface of the opposite polaritygenerally facing away. Preferably, the poles of rotor 110 are disposedsuch that with current flowing in one direction in field coil 116, thepoles of rotor 110 alternate in magnetic polarity in the followingmanner:

                  TABLE 5                                                         ______________________________________                                        Pole           Type   Polarity                                                ______________________________________                                        118            PM     north                                                   130            PM     south                                                   120            WF     north                                                   132            WF     south                                                   122            PM     north                                                   134            PM     south                                                   124            WF     north                                                   136            WF     south                                                   126            PM     north                                                   138            PM     south                                                   128            WF     north                                                   140            WF     south                                                   ______________________________________                                    

With current flowing in this direction in field coil 116, the voltageinduced by the wound field poles "boosts" the voltage induced by thepermanent magnet poles. With the current in field coil 116 reversed, thepoles have the following magnetic polarity:

                  TABLE 6                                                         ______________________________________                                        Pole           Type   Polarity                                                ______________________________________                                        118            PM     north                                                   130            PM     south                                                   120            WF     south                                                   132            WF     north                                                   122            PM     north                                                   134            PM     south                                                   124            WF     south                                                   136            WF     north                                                   126            PM     north                                                   138            PM     south                                                   128            WF     south                                                   140            WF     north                                                   ______________________________________                                    

With current flowing in this direction in field coil 116, the voltageinduced by the wound field poles "bucks" the voltage induced by thepermanent magnet poles.

Current control for an alternator comprising rotor 10' can beaccomplished with the bi-directional voltage regulator 80 of FIG. 4.

The permanent magnets which replace the wound-field poles in thisembodiment of the present invention can be attached to pole pieces 112and 114 in a variety of ways. For example, in forging pole pieces 112and 114, a thin axially-projecting ledge can be formed in place of eachof the wound-field poles which are to be replaced. The permanent magnetscan then be attached on the ledges with a suitable method, such as withadhesive or with KEVLAR banding. Alternatively, in forging pole pieces112 and 114, pockets can be formed in the bodies of pole pieces 112 and114 where permanent magnets are to be employed instead of wound-fieldpoles. The permanent magnets can each then be inserted with one end in apocket and the other end extending axially, in the same way that thewound-field pole fingers extend.

Various other modifications and variations will no doubt occur to thoseskilled in the arts to which this invention pertains. Such variationswhich generally rely on the teachings through which this disclosure hasadvanced the art are properly considered within the scope of thisinvention. This disclosure should thus be considered illustrative, notlimiting; the scope of the invention is instead defined by the followingclaims.

What is claimed is:
 1. An electrical machine comprising:a generally-annular stator; and a rotor rotatably mounted within said stator and defining an axis of rotation, said rotor having an axially-extending periphery, said rotor comprising a plurality of wound-field poles disposed about at least one portion of said periphery and a plurality of permanent magnet poles all disposed about at least one circumferentially-different portion of said periphery, said wound-field poles in general axial alignment with said permanent magnet poles, said permanent magnet poles each further having a magnetized surface of one polarity generally facing said stator and a magnetized surface of the opposite polarity generally facing away from said stator; wherein said wound-field poles are salient poles; wherein said permanent magnet poles are disposed in circumferentially-adjacent pairs, without an intervening wound-field pole; wherein each said circumferentially-consecutive pair of permanent magnet poles share a common radially-extending base; and wherein said wound-field poles are disposed in circumferentially-consecutive pairs, without an intervening permanent magnet pole.
 2. An electrical machine as recited in claim 1, wherein said pairs of circumferentially-adjacent permanent magnet poles are of opposite magnetic polarity and said pairs of circumferentially-adjacent wound-field poles are of opposite magnetic polarity.
 3. An electrical machine as recited in claim 2, wherein:said wound-field poles each have a base with windings wound around said base; and said bases of said circumferentially-adjacent pairs of wound-field poles are substantially parallel.
 4. An electrical machine as recited in claim 3, wherein said rotor comprises six wound-field poles and six permanent magnet poles.
 5. An electrical machine comprising:a generally-annular stator; and a rotor rotatably mounted within said stator and defining an axis of rotation, said rotor having an axially-extending periphery, said rotor comprising a plurality of wound-field poles disposed about at least one portion of said periphery and a plurality of permanent magnet poles all disposed about at least one circumferentially-different portion of said periphery, said wound-field poles in general axial alignment with said permanent magnet poles; wherein said wound-field poles are disposed in circumferentially-consecutive pairs.
 6. An electrical machine as recited in claim 5, wherein said permanent magnet poles are disposed in circumferentially-consecutive pairs.
 7. An electrical machine as recited in claim 6, wherein said pairs of circumferentially-consecutive permanent magnet poles are of opposite magnetic polarity and said pairs of circumferentially-consecutive wound-field poles are of opposite magnetic polarity.
 8. An electrical machine as recited in claim 7, wherein:said wound-field poles are salient poles; said wound-field poles each have a base with windings wound around said base; and said bases of said circumferentially-consecutive pairs of wound-field poles are substantially parallel.
 9. An electrical machine as recited in claim 8, wherein said rotor comprises six wound-field poles and six permanent magnet poles.
 10. An electrical machine as recited in claim 7, wherein:said rotor further comprises two pole pieces each with generally-circular bodies, the bodies of said pole pieces defining an axis of rotation of said rotor; and at least some of said wound-field poles extend axially from one of said pole pieces. 